In some fuel cells, instead of a pure fuel component, diluted fuel is used which will be designated below as fuel mixture, even if not all components of this mixture are oxidisable substances.
In a direct methanol fuel cell (DMFC), the fuel mixture on the anode side consists, for example, of methanol dissolved in water, the first being the actual fuel. The water of this fuel mixture does not appear in the net accounting equation of the cell reaction, as distinguished from the water arising as reaction product (on the cathode side), which has to be discharged from the cell as the reaction product carbon dioxide arising on the anode side. That is, the anode fluid undergoes a depletion of methanol and an enrichment of CO2 on its path from the anode inlet to the anode outlet. In order to be able to optimally utilize the depleted anode fluid, too, and to avoid liquid losses, as a rule, a circuit flow is provided on the anode side, wherein the anode fluid is again enriched with fuel (corresponding to its consumption) after it has left the anode outlet and fed to the anode inlet again. In the process, however, carbon dioxide has to be discharged from the circuit flow.
In the DMFC system, the major proportion of carbon dioxide is present in a gaseous form as the solubility limit of carbon dioxide in the fuel mixture is quickly exceeded. (As water is quantitatively the dominant substance in the fuel mixture, the solubility limit of carbon dioxide in the fuel mixture approximately corresponds to that of carbon dioxide in water.) That is, the fluid exiting at the anode outlet is as a rule no homogenous phase but a gas/liquid mixture. Due to the flow conditions, however, the liquid and the gaseous phases are not physically strictly separated from one another; gas bubbles are rather formed in the liquid.
In the DMFC system, the liquid phase exiting at the anode outlet is a water/methanol solution, as a rule saturated with CO2; in the gaseous phase, CO2 enriched with water vapour and methanol vapour is dominant. Thus, in an unregulated waste gas removal, the fuel (here: methanol) present in the gaseous phase would be thus lost for the system, which is unacceptable not only for economical, but also for health and safety reasons. Furthermore, water in the form of water vapour would be lost, so that for maintaining the operation conditions, an external water supply would be necessary, which is unacceptable with respect to the practical use of the fuel cell.
The above-mentioned problems have to be taken into consideration in the conception and operation of a DMFC system, which is conventionally done as described below.
FIG. 1 is a view of a typical fuel cell system according to the present internal prior art. The DMFC fuel cell is schematically and only for simplification divided into a cathode side K and an anode side A. (“Side” is not to be understood figuratively: in fact, a DMFC fuel cell consists, as a rule, of a so-called stack with alternating anode and cathode areas).
The fluid on the cathode side comprises an oxidising substance, such as oxygen, which is supplied by means of a metering device 1 in the form of normal ambient air.
In the process, the non-usable substances of the air, such as nitrogen, but also water arising as reaction product and CO2 diffusing from the anode to the cathode side, are discharged as fluid 21 at the outlet.
The fluid on the anode side further comprises, apart from methanol, the substances water and carbon dioxide, the latter having to be discharged from the fuel cell as waste gas 15.
In the system shown in FIG. 1, two separate separating devices are provided which separate gas from the fluid 11 discharged at the anode outlet for separating CO2 or and, respectively, recover water corresponding to the losses on the anode side from the fluid 21 discharged at the cathode outlet.
The recovered water 14 or the water/methanol mixture 13, respectively, is again supplied to the anode inlet of the fuel cell in a reflux 12 (by means of a pump 2), methanol M being admixed from a storage tank T via a metering pump 3. The purified waste gases 15, 16 (CO2, dried exhaust air) are discharged to the surroundings.
One of the essential problems is to keep the amount of water contained in the system as constant as possible, so that the necessity of a separate water supply can be avoided. As water in the form of water vapour can be discharged with the substances to be discharged on the anode side as well as with the waste gases (“exhaust air”) on the cathode side, the latter not only comprising product water (to be discharged quantitatively), but also water to be recycled to the anode side which flows from the anode to the cathode side due to the “water drag” effect, the maintenance of a constant amount of water in the fuel cell is very elaborate.